A technique for synthesizing diamonds out of the vapor-phase was started by Derjaguin (USSR Inv. Certif. No. 339134, 1958) in 1956 and by Eversole (USP Nos. 3,030,187 and 3,030,188) in 1958, and has been vigorously developed since 1982, based on a study made by Matsumoto et al. (Japanese Journal of Applied Physics, Vol. 21, 1982, L183 to L185).
A microwave plasma CVD process of synthesizing a diamond in plasma excited by microwaves was invented by Kamo et al. (Journal of Crystal-Growth, 62, 1983, pp. 642-644) in 1983. According to this process, it is possible to obtain a film containing no impurity due to an electrodeless discharge. Thus, the cost for such a film has been reduced by an increase in the formation area and by an increased growth rate.
Such a conventional method of vapor-phase synthesizing diamond in plasma has been basically carried out through the so-called open type apparatus which does not circulate any material gas, as shown in FIG. 1. Referring to FIG. 1, a raw material gas supply system 1 supplies raw material gas, which is prepared by mixing about 1 to 2% of carbon source gas with about 99% of medium gas, into a reaction tube 3 through a raw material gas supply valve 2. The carbon source gas is prepared from hydrocarbon such as methane, ethane, propane, acetylene or benzene, alcohol such as methanol, ethanol or propanol, organic acid such as acetic acid, ketone such as acetone, ester such as methyl acetate, carbon monoxide, carbon dioxide, or the like. The medium gas is prepared from hydrogen or oxygen. The carbon source gas and the medium gas are reacted in the plasma for synthesizing diamond. Merely 1 to 2% of the carbon source gas employed for such reaction is converted into diamond in practice. The raw material gas, including unreacted carbon source gas, once subjected to the reaction is entirely exhausted by an exhaust system 5 through an exhaust valve 4. Further, the pressure in a vacuum vessel is regulated by controlling the raw material gas supply system 1 and the exhaust system 5. Plasma generation means 6 generate the plasma in the reaction tube 3. This plasma generation means 6 includes a microwave oscillator 7 and a waveguide 8. In general, the microwave oscillator 7 generates microwaves at a frequency of 2.45 GHz, while a for synthesizing diamond, while a tuner 10 and a plunger 11 adjust the waveguide 8, which is monitored by a power monitor 9, in order to conform the same to the frequency of the microwaves.
It has been confirmed that it is possible to obtain a film type or thin-plate type diamond of high quality, the performance of which is equivalent to that of natural diamond or bulk type diamond synthesized by a superhigh pressure synthesizing method, in accordance with diamond vapor-phase synthesis, particularly the microwave plasma CVD process, by restricting synthesis conditions. More specifically, it has been possible to obtain diamond having a Vickers hardness of 10000, which is equivalent to that of a single crystal of diamond. Further, it has also been possible to attain a thermal conductivity of 16 W/cm.multidot.K, which is not necessarily inferior to the value 22 W/cm.multidot.K of a single crystal type IIa diamond and the value 11 W/cm.multidot.K of type Ia diamond. Further, a heat sink or a rear portion is used in the form of a plate-type film or a film provided on a base material. Thus, it is possible to considerably save time for working according to the vapor-phase synthesizing method, which is adapted to already define a film or a plate having a thickness of several to hundreds of millimeters in film formation. However, a step of cutting diamond in the form of a plate by laser cutting or the like is required when single-crystalline diamond is employed. In the existing circumstances, however, sufficient results cannot be obtained as to the vapor-phase synthesized diamond, due to cost problem. To this end, cost reductions are of interest in every aspect, in relation to an extension of the formation area and an increase in the growth rate. In order to extend the formation area in the microwave plasma CVD process, efforts have been made to improve the methods of generating plasma in various ways.
Generally, the formation area is extended by the following two means: The first means employs an apparatus using a large-diameter microwave plasma process, as shown in FIG. 2. According to the first means, the effect of the introduced microwaves are enlarged by a tapered waveguide 21 so that plasma 23 is generated in a bell jar 22 by the microwaves, thereby vapor-phase synthesizing a diamond film on a wafer 26 provided on a holder 25 which is rotated by a rotation drive mechanism 24. According to this first means, the wafer 26 is made of a silicon substrate of 4 inches in diameter, and the growth rate is about 0.2 .mu.m/hr.
The second means employs an apparatus using an ECR plasma process, as shown in FIG. 3. According to this second means, electromagnetic coils 33 apply a magnetic field to the microwaves for increasing the impingeable surface area whereby the microwaves travel from a waveguide 31 having a small rectangular sectional area, into a trapezoidal cylindrical waveguide 32. Thus, ECR plasma 36 is generated in the vicinity of the surface of a substrate 35 which is heated by a heater 34, thereby synthesizing a diamond film out of the vapor phase on the surface of the substrate 35. When this means is employed, the substrate 35 is prepared from a silicon substrate of 2 inches in diameter, with a diamond film growth rate of about 0.1 .mu.m/hr.
When the aforementioned open type vapor-phase synthesizing apparatus is employed, however, the film quality is reduced in the means for extending the film forming area shown in FIGS. 2 and 3. Although attempts have been made to increase the pressure the, concentration of fuel gas and the like in order to increase the growth rate, in particular, no success has been attained due to the instability of the plasma and due to deposition of a non-diamond component such as graphite. While an attempt has also been made to rotate the substrate in order to homogenize the plasma, portions of relatively good quality overlap with defective portions, whereby the film quality as a whole is reduced. Further, the film quality tends to be lowered in film areas away from the central portion of the plasma. The just described techniques are still in the stages of development. According to the above described methods, the growth rate is generally limited to about 1 .mu.m/hr. under pressure of 40 Torr. Area increase and high-speed film formation are limited, and values of at least 4 inches in diameter and about 10 .mu.m/hr. are regarded preferable. The conventional open type apparatus cannot possibly attain such values.
Other problems of the conventional open type vapor synthesizing apparatus for vapor-phase synthesizing diamond are as follows:
Merely a few percent of the supplied carbon source gas is subjected to reaction, and the remaining part thereof is exhausted from the system in an unreacted state. Further, the medium gas, which is required to occupy approximately 99% of the overall raw material gas, is finally exhausted in an unreacted and unchanged state. In other words, the known apparatus requires the raw material gas in an amount 5000 times that of the carbon source gas which is converted into diamond in practice. Thus, the material expense takes the greater part of the cost for diamonds which are vapor-phase synthesized at a relatively small equipment cost. Namely, it is necessary to effectively use the raw material gas, in order to reduce the material expense.
On the other hand, it has been confirmed that fluctuations in the pressure and in the raw material composition, exert bad influence on the synthesis of a high quality film. Further, valves of the supply and exhaust systems must be operated in a complicated operation for regulating the pressure and the raw material composition, and it is difficult to stabilize the conditions for a long-time operation. Thus, the valves must be finely controlled by an operator, leading to a high cost. If such pressure regulation is automated, the equipment cost is substantially increased to raise the overall costs. Hence, it is also necessary to simplify the valve operation and pressure regulation, in order to implement an automated operation.
In order to solve these problems, a chemical reactor may be sealed to circulate raw material gas and supply carbon source gas only for an amount consumed for synthesizing diamond, thereby effectively using the raw material. Further, it is conceivable that no pressure regulation is required in such a sealed system since the reaction space is sealed, and hence a valve operation can be simplified. However, in such a reaction wherein a carbon source gas is decomposed in a plasma to generate diamond, the following reaction (1) generally progresses if the carbon source gas is made of a hydrocarbon, for example: EQU C.sub.x H.sub.y .fwdarw.xC+y/2H.sub.2 ( 1)
Due to such a reaction, the pressure in the sealed reaction vessel is increased by the generated hydrogen gas to exert an influence on the plasma state whereby the film quality deteriorates, except for the case of y=2. Further, the carbon source gas itself is a bulk as expressed in the following formula (2), to cause similar problems: EQU nC.sub.x H.sub.y .fwdarw.C.sub.nx H.sub.ny-.alpha. +.alpha./2H.sub.2( 2)
Thus, it is difficult to stably control the raw material gas concentration or to avoid a pressure increase by merely sealing the reaction tube 3 of the conventional open type apparatus for vapor-phase sinthesis.